A thermal fluid circulator in the context of the invention is a device or system that is commonly used in the plastics, metals, ceramics, and die cast molding industries in the control of operating temperatures of molds, dies or injection barrels. Circulator systems may incorporate a built-in chiller. Central chilling systems need a main circulator pumping system. Alternatively, they are also built using water tower evaporative cooling technology. Circulators are also used in other industries besides plastics for similar temperature control purposes. Sometimes circulators are used for controlling the operating temperatures in a two component mixing process such as molding liquid silicone rubber or LSR and sometimes called LIM, which is exothermic process where heat is given off when the polymer chains are cross linking. This type of process requires precise temperature control of a specially designed injection barrel which keeps the two part mixture from chemically setting up prematurely.
The circulator is spoken about in the trades by different names. It is sometimes called a temperature control unit or “TCU”. It is called a “Thermolator” in some areas. It may also be called a “water circulator.” Some circulator units use oil as the thermal fluid medium and are called “oil circulators or TCU's.” Oil circulators are primarily used to heat, not cool the mold, die or barrel. Water circulators can circulate water over a wide range of temperatures depending on system pressure. Under higher than ambient pressures, water-based systems typically are used for heating up to 300° F. and can be configured to go up as high as to 500° F., and are commonly used where heat is desired instead of cooling for the molding of thermoset plastics or just high temperature plastics.
Circulators come in two basic schemes. One type of system is called “direct injection” and the other is called “closed loop.” “Direct injection” and “closed loop” describe how the fluid that is directed from the fluid pump to the mold, die or barrel, is returned to the main circulation system after absorbing or shedding thermal energy from the molding process. Circulators can be configured to be both styles and convertible from one or the other in the field.
The amount of energy absorbed or shed by the thermal fluid depends on several variables including the mold details, the process details, and the thermal fluid temperature control (circulator) system details. With regard to the mold, for example, variables may include the thermal conductivity of the material of which the mold, die or barrel is fabricated, the volume (mass) of the mold, the amount and temperature of the material being molded, the amount of surface area exposed to localized and/or total ambient air temperatures and to other incidental or purposeful environmental heating and cooling influences affecting the mold. With regard to the process, variables include its location and concentration within the mold, range of variation or curve of thermal demand or excess over a cycle of operation, duration of the thermal cycle, and dwell time between cycles. With respect to the temperature control (circulator) system, relevant variables include the proximity of the process within the mold to the network of thermal fluid channels, the absolute and average temperature differential of the fluid and the process; the absolute and average rate of BTU transfer required to sustain a repetitive or continuous process, the thermal conductivity and capacity of the thermal fluid, the volume and surface area of the fluid channels within the mold, and the time of exposure and flow rate of the fluid within the mold. The temperature control (circulator) system must have the capacity to sense the thermal requirement, and to supply and control a sufficient quantity of thermal transfer fluid at the right temperature and rate to satisfy the requirement.
Using an example of a water type circulator being used to control the temperature of a plastics injection mold; the direct injection technique directly adds cooled water and extracts heated water to and from the “loop of water” circulating between the pump and the mold in order to control the loop of water temperature and thus in turn the temperature of the mold. In this example, mold or process temperature is controlled, at least in part, by measuring and comparing the temperature of the water returning from the mold against a set point to obtain the temperature rise. The addition of cool water is used to maintain the control loop temperature to a desired set point.
In an alternative technique called “closed loop,” the “loop of water” or fluid circulation path passes through a water to water heat exchanger to remove the excess heat picked up from the molding process. Coolant is cycled on and off to the heat exchanger to control the control loop temperature.
Regardless of which scheme is used, the primary method for generating the fluid flow in the current technology conventional circulator is to use a standard, fixed speed electric motor mated to a centrifugal pump. However, with a conventional centrifugal pump circulator there is no means of controlling the pump pressure during molding operations. The temperature control of the loop water or fluid is the only control mechanism available for dynamic control of the mold temperature. While this form of process temperature control is simple and adequate for many purposes, it lacks consideration and control of other variables affecting the cooling dynamic. Voltage variations and slippage in the centrifugal pump affect the rate and volumetric throughput of thermal fluid in the mold, and ultimately reduce the precision of process temperature control.
Additionally, fluctuations in the supply voltage to the motor will likely affect motor speed and system pressure, altering the balance of variables in the cooling dynamics. These pressure fluctuations affect the rate of flow and the precision of process control. This results in a less than optimal design for a circulator system, and does not yield efficient and precise control of the mold temperature. Neither “direct injection” nor “closed loop” modes of circulators using conventional fixed speed/centrifugal pump systems can offer programmable flow schemes controlling more than fluid temperature.
In the field of plastics molding, die casting, metal molding, and ceramic molding there are temperature controllers that use intermittent or pulsed flow techniques. This method for controlling mold die or barrel temperatures is commonly called pulsed cooling or pulsed tempering in the trades. In pulsed cooling schemes the flow is metered in an intermittent on/off method using valves. The “on” duration is lengthened or shortened to compensate for changes in supply pressure and of mold, die or injection barrel temperature. This requires a temperature sensor in the mold or mold water return line just as it exits the mold frame to sense the temperature fluctuations in the process fluctuations, rather than the water loop sensor described previously. Pulsed flow techniques have been disclosed by others, such as in basic injection molding practice, how a controller advances the mold cooling control technology through the use of turning valves on and off or as it is called in the trades, “pulsed cooling” or “pulse tempering”. These control systems accomplish their control though the use of a valve that is opened or closed. The controller has a means of measuring the either mold steel temperature, the overall cycle time, or the exit water temperature with appropriate sensors, to determine how often the valve needs to be pulsed on during the injection cycle to maintain a target temperature set point.
What is needed, therefore, are improved systems and techniques for process temperature control in an injection molding process.